Remote sensing, i.e. the monitoring of particular gas molecules in the Earth's atmosphere, started with instruments having single element sensors in front of which a spectral band filter was placed, e.g. TOMS. In order to arrive at some sort of Earth coverage, a 1D scanner in swath direction was required. In flight direction the coverage was obtained by the relative velocity of the satellite with respect to the Earth surface. Later these single detectors were replaced by linear arrays. These were used in a similar way as the single cell detectors, except that no band filter was used. Instead of that, a spectrometer was employed to spread out the light over the number of pixels of the linear array. Finally, 2D sensors became available so that the swath scanner was no longer needed. The full swath is imaged onto a row of pixels while each ground pixel is spectrally dispersed in the orthogonal direction.
Satellites are orbiting the Earth at an altitude of typically between 400 and 800 km. These orbits are indicated as Low Earth Orbits (LEO). The speed of these satellites relative to the Earth is about 7 km/s. Since an integration time of about 1 s is needed to arrive at a good enough Signal to Noise Ratio (SNR), an effective ground pixel on Earth will be about 7 km in flight direction. Most often some binning in the swath direction is used leading to square ground pixels of e.g. 7×7 km.
The drawback of a spectrometer is that the intensity being measured is dispersed over many spectral detector pixels resulting in low signal levels per detector pixel.
Scientists are asking for smaller ground pixels while keeping good SNR values. This is virtually impossible since the scattering by the Earth is a constant, as is the output of the Sun, so the only way to decrease the ground pixel size is by moving towards larger entrance apertures and smaller f-numbers in the optical design. This leads to larger, heavier, and more expensive instruments.